1. Field of the Invention
This invention relates to an injection-molded article which is improved in external appearances upon being molded and upon being painted while maintaining a good balance between low temperature impact resistance and flexibility, and more particularly to a polymer material suitable for a large-sized exterior part of an automotive vehicle.
2. Description of the Prior Art
In recent years, a tendency of making automotive parts of plastic is rapidly progressing in order to lighten the weight of an automotive vehicle for the purpose of improving fuel economy. In this regard, a variety of plastics have been used as the material of automotive parts such as a bumper, a facia, a door garnish and parts of a vehicle body which are conventionally formed of metal, in addition to an instrumental panel, a console box, a glove box, a steering wheel, interior parts such as a trim member, a molding protector, a lamp housing, a front grille, a mud guard and a corner bumper.
Examples of plastic as the material of the above automotive parts are RIM (Reaction injection Molded) polyurethane, composite polypropylene, and a so-called polymer alloy made of polycarbonate/ABS or polyphenylether/polyamide.
Concerning the material or plastic to be used for a bumper or a facia, it is classified to two types. One of them is a plastic having such a characteristic as to mainly absorb impact of a vehicle collision upon being deformed and thereafter recover the original shape. The other is a plastic which is lower in deformation recovery characteristics but higher in rigidity than the above plastic, while being light in weight and inexpensive.
Examples of the former plastic are RIM polyurethane, polypropylene or ethylene-propylene copolymer rubber which is blended with partially cross-linked ethylene-propylene copolymer rubber, polypropylene blended with oil. These generally exhibit a flexural elastic modulus of about 2000 to 5000 kg/cm.sup.2. Examples of the latter plastic are ethylene-propylene copolymer rubber and polypropylene blended with inorganic filler. These exhibit a flexural elastic modulus of about 8000 to 12000 kg/cm.sup.2. The above-mentioned plastics except for RIM polyurethane have been extensively used because of their capability of being fabricated by injection molding at a high productivity and at a low production cost.
As discussed above, a low rigidity type bumper has been usually formed of the material (plastic) such as polypropylene blended with a partially cross-linked ethylene-propylene copolymer rubber or polypropylene blended with oil, of the above-listed materials. These are produced by methods disclosed, for example, in Japanese Patent Provisional Publication Nos. 53-145857, 54-16554 and 57-135847. Another material (plastic) for a low rigidity type bumper is polypropylene-based composition blended with a noncross-linked ethylene-propylene copolymer rubber and a high density polyethylene. This plastic is produced by a method disclosed, for example, in Japanese Patent Provisional Publication No. 56-106948. A further material (plastic) for the same type bumper is a polypropylene composition blended with a noncross-linked ethylene-propylene copolymer rubber and oil. This material is produced by a method disclosed, for example, in Japanese Patent Provisional Publication No. 63-146951. A high rigidity type bumper has been usually formed, for example, of polypropylene composition blended with a noncross-linked ethylene-propylene copolymer rubber which is produced by a method disclosed, for example, in Japanese Patent Provisional Publication Nos. 52-23148 and 52-126450.
However, difficulties have been encountered in the above-discussed materials (plastics) particularly in cases where the materials are used for a large-sized injection-molded article such as an automotive bumper which is so shaped as to correspond to a vehicle body in order that the surface thereof is aligned with that of the vehicle body. In other words, the large-sized article such as the bumper formed of the above-discussed materials exhibits a coefficient of linear expansion of about 10 to 17.times.10.sup.-5 /.degree.C. which is considerably high as compared with that of steel sheet of the automotive body. Accordingly, the dimension of the bumper changes with ambient temperature change (from -40.degree. to 80.degree. C.), so that the surface alignment characteristics of the bumper with the automotive body does not occur. As a result, the above-discussed plastics cannot be used as the material for the bumper requiring the surface alignment characteristics with the automotive body. In view of this, it is now common that such an automotive part is formed of an expensive polyurethane-based material containing fibrous filler having a low coefficient of linear expansion.
In addition, the above-discussed materials for the low rigidity type bumper, produced by the methods disclosed in the above Japanese Patent Publications require a complicated compounding process and/or oil extending operation in their production, so that there is a tendency for the flow of the material to be irregular. As a result, the thus produced materials for the low rigidity type bumper necessarily cannot provide an injection-molded article which is good in external appearance and low temperature impact resistance.
The above-discussed materials for the high rigidity type bumper, produced by the methods disclosed in the above Japanese Patent Publications have a good balance among heat resistance, moldability and the like but is high in rigidity which has inferior in impact energy absorbing characteristics during a vehicle collision and in deformation recovery characteristics after the vehicle collision. Additionally, these materials are high in coefficient of linear expansion similarly to the above materials for the high rigidity type bumper, and accordingly not satisfactory in surface alignment of the bumper with the automotive body.
As a measure of producing a material (plastic) having a low coefficient of linear expansion, it is common to fill sheet-shaped or fibrous filler into the above-discussed materials or compositions. Glass fiber, talc, whisker and/or the like is used as the filler as disclosed, for example, in Japanese Patent Provisional Publication Nos. 61-21145 and 63-57653. It is to be noted that a large amount of filler is required to be filled in the material in order to obtain a sufficiently low coefficient of linear expansion. However, filling such a large amount of filler unavoidably lowers the flowability of the material and degrades the external appearance of a molded article in connection with weld marks and that of a painted article in connection with the gloss and brightness character, while providing an insufficient low temperature impact resistance.